Touchpad has been widely used in various electronic products, for example notebook computer, personal digital assistant (PDA), mobile phone and other electronic systems. Touchpad serves as an input device where users could touch or slide thereon by an object, for example finger or fingers, to control the cursor on a window in relative movement or absolute coordinate movement to support various input functions such as text writing, window scrolling and button pressing. Conventionally, the sensor of a touchpad has symmetrical structure such as the square shape shown in FIG. 1. The traces of the touchpad sensor all have same shape and area, and thus the base capacitances of the traces are symmetrically distributed across the touchpad sensor. The sensed capacitive values caused by an object touching on the touchpad sensor are also symmetrical and linear across the touchpad sensor as shown in FIG. 2. However, the shape and structure of a touchpad sensor would be changed with different applications and produces asymmetrical sensing characteristics accordingly. An asymmetrical touchpad sensor refers to one including at least one of the features of the touchpad sensor, such as the shape of the sensor, the thickness of each sensing layer in the sensor, the area of the traces, and the distances between the traces to the grounding layer, that is asymmetrical. In a touchpad sensor, the base capacitance of a trace is proportional to the area of the trace and the inverse of the distance between the trace and the grounding layer, or simply represented byC=ε×(A/d)  (Eq-1)where C is the base capacitance of the trace, ε is the dielectric constant, A is the area of the trace, and d is the distance between the trace and the grounding layer. The sensed capacitive value of the trace caused by an object isS∝(ΔC/C)  (Eq-2)where ΔC is the differential capacitance of the trace caused by the object. Therefore, the area of the trace and the distance between the trace and the grounding layer both are factors of determining the base capacitance of the trace. For example, in a circular touchpad sensor 100 shown in FIG. 3, the traces X0 to X6 along the horizontal direction have different lengths and different areas. From the equation Eq-1 it is conducted that, if all the traces of a touchpad sensor are spaced from a grounding layer with a same distance, the trace having greater area will have greater base capacitance. Accordingly, the base capacitances of the group of traces X0 to X6 and the group of traces Y0 to Y6 are asymmetrically distributed across the touchpad sensor 100. As illustrated by the equation Eq-2, when an object operating on the touchpad sensor 100, the sensed capacitive value S will vary with position across the touchpad sensor 100 as shown in FIG. 4, since the traces X0 to X6 and Y0 to Y6 of the touchpad sensor 100 have different base capacitances therebetween. The asymmetricity and nonlinearity of the sensed capacitive value S will cause the touchpad having misjudgment to an operation or undesired offset in the judged position to a touch of an object operating thereon.
Therefore, it is desired a compensation to the sensed capacitive values of a touchpad sensor.